A magnetic resonance system is a device which transmits a radio-frequency electromagnetic field (RF) of a specific frequency to an object placed in a static magnetic field so as to induce a magnetic resonance phenomenon, and thereby acquires physicochemical information about the object.
Magnetic resonance imaging (MRI), which is now widely used, is a method of imaging the proton density and the difference of relaxation time which varies within biological tissues, mainly by using the magnetic resonance phenomenon of the protons in water molecules. A method known as spectroscopic imaging has also been proposed. This is a method of separating the magnetic resonance signals for each molecule based on the difference in the magnetic resonance frequency (chemical shift) due to the difference in the chemical bonds of a molecule, and imaging the concentration and relaxation time for each molecular species.
In the prior art, to add spatial information required for this imaging, a gradient field was applied to generate a signal whose frequency changes with position. With the spin echo method or the gradient echo method which are typical imaging methods, repeated measurements are required while varying the phase encode gradient. Therefore, there was the problem that the measurement time was too long. When the measurement time is long, there is a problem of generating motion artifacts and degrading image quality due to movement of the circulatory organs, breathing apparatus and alimentary canal of the object, and also, in diffusion imaging which measures the molecular diffusion of molecules and blood flow imaging which measures blood flow, a precise value could not be obtained.
In order to resolve these issues, measurement methods which modify the way in which the RF field or gradient is applied have been proposed. For example, there is fast SE which acquires multiple echoes at once by applying plural inversion pulses of an RF field, and echo planar which acquires multiple echoes at once using an oscillating gradient which reverses in a short cycle.
On the other hand, various methods have been proposed wherein multiple RF coils are provided, and spatial information is added using the differences in the sensitivity maps of the coils concerned.
In Patent document 1, a measurement method is proposed for using multiple RF coils without coupling. Thereby, for example, simultaneous measurement of parts which are considerably distant from each other, such as the head and the heart, can be performed.
In JP-A No. 1999-206735, a method is proposed for imaging the intersection part of the sensitivity region during transmit and the sensitivity region during receive by sequentially using multiple RF coils disposed on the circumference. This is supposed to make the gradient in the circumferential direction unnecessary.
In JP-A No. 2004-201756, an RF coil which comprises multiple RF coils and a switch for changing them over, is proposed.
In JP-A No. 2005-152657 and JP-A No. 2005-152655, a method is proposed for simultaneously transmitting RF from multiple coils to control of the shape of the excitation region and decreasing the SAR (Specific Absorption Rate).
In JP-A No. 2002-153440, a method is proposed for secondarily adding spatial information by sensitivity maps using multiple receive RF coils. In this method, the number of phase encodes is reduced, and aliasing artifacts produced thereby are removed using differences in the sensitivity maps of the RF coils.
In JP-A No. 1997-262219, a method is proposed for using one RF coil, wherein the shape of the RF field is a shape known as a DANTE pulse, a comb-like region is selectively excited, and the measurement time is shortened.
In McDougall M P, Wright S M, 64-channel array coil for single echo acquisition magnetic resonance imaging. Magnetic Resonance in Medicine, 2005, No. 54, pp. 386-392, a construction is proposed wherein multiple RF coils having thin slices as a sensitivity map are disposed in an array. For example, when the direction of the array is the Y direction, it is proposed that imaging is possible without using a phase encode gradient in the Y direction at all.